Synthetic vascular grafts are routinely used to restore the blood flow in patients suffering from vascular diseases. For example, prosthetic grafts made from expanded polytetrafluoroethylene (ePTFE) are commonly used and have shown favorable patency rates, meaning that depending on a given time period, the graft maintains an open lumen for the flow of blood therethrough. Vascular grafts formed of ePTFE include a microstructure characterized by spaced apart nodes connected by fibrils, the distance between the nodes defined as internodal distance (IND), and are generally extruded either as a tube or as a sheet or film that is fashioned into a tube, but can also be created from fibers woven or knitted into a generally tubular shape.
Vascular grafts can be produced for use as a conduit, such as to bypass a damaged or diseased portion of a blood vessel, or for attachment to a stent or other intraluminal structure as a covering. A vascular graft covering may be positioned on one or more surfaces of an intraluminal structure to impart desired characteristics thereto. For example, a graft covering on an inner wall of a stent enables the smooth flow of blood through the stent graft when deployed in a blood vessel. Due to the relatively small dimensions of blood vessels and bodily cavities, a minimal profile of an intraluminal prosthesis, such as a stent graft, is important. As a result, it is desirable to provide a graft or covering with a uniformly thin wall.
The formation of a tubular ePTFE graft or covering using a ram extrusion process, followed by heating and longitudinal stretching, is well-known. For example, U.S. Pat. No. 6,620,190 to Colone describes extrusion processes for producing ePTFE tubular structures. According to conventional ram extrusion techniques, typical wall thicknesses for pre-stretched polytetrafluoroethylene (PTFE) tubes (i.e., “green tubes”) range from approximately 0.250 mm to approximately 1 mm. However, U.S. Patent Application Publication No. 2003/0082324 to Sogard et al. describes an extrusion process that allegedly yields a PTFE tube extrudate of between about 0.100 mm to about 0.250 mm. This is accomplished, at least in part, by injecting air into the lumen of the extrudate as it exits the extruder die to prevent collapse, which is further described in U.S. Pat. No. 6,814,561 to Venditti et al. and is incorporated by reference in its entirety into this application. As mentioned, following extrusion, the PTFE green tube is stretched to produce a porous ePTFE structure. Stretching techniques, such as those described in U.S. Pat. No. 4,177,334 to Okita, can be used to thin the walls of extruded PTFE tubes. However, difficulties in the formulation of an extruded ePTFE tube with a uniformly thin wall are reported in U.S. Pat. No. 5,700,285 to Myers et al., which alternatively describes the use of wrapped ePTFE films to achieve a desired wall thickness of less than about 0.10 mm.
References describing extrusion of PTFE green tubes and/or the formulation of ePTFE products include: U.S. Pat. No. 3,295,166; U.S. Pat. No. 3,953,566; U.S. Pat. No. 4,145,385; U.S. Pat. No. 4,177,334; U.S. Pat. No. 4,208,745; U.S. Pat. No. 4,225,547; U.S. Pat. No. 4,250,138; U.S. Pat. No. 4,482,516; U.S. Pat. No. 4,743,480; U.S. Pat. No. 4,876,051; U.S. Pat. No. 4,925,710; U.S. Pat. No. 5,026,513; U.S. Pat. No. 5,505,887; U.S. Pat. No. 5,620,763; U.S. Pat. No. 5,700,285; U.S. Pat. No. 5,735,892; U.S. Pat. No. 5,810,870; U.S. Pat. No. 5,874,032; U.S. Pat. No. 6,048,484; U.S. Pat. No. 6,436,135; U.S. Pat. No. 6,530,765; U.S. Pat. No. 6,814,561; and U.S. Patent Application Publication No. 2003/0082324, each of which is incorporated by reference in its entirety into this application.
Applicants have recognized that it would be desirable to provide an apparatus for use with a conventional ram extruder, which extrudes a strong coherent tubular member with an extremely thin wall. Applicants have also recognized that it would be desirable to provide a method for producing strong coherent extremely thin-walled tubular members using conventional extrusion techniques.